The present invention relates to high pressure fuel supply pumps and in particular to damping pressure pulses arising from the reciprocation of a pumping piston that pressurizes fuel for discharge from such pump.
An increasing number of fuel systems for on-road vehicles are adopting the so called “common-rail” configuration, whereby a fuel pump maintains a reservoir or “rail” at a pressure at or above that required for fuel injection, and a plurality of injectors are in fluid communication with the rail via respective injection valves that are electronically actuated, thereby controlling the timing and quantity of fuel delivered from the rail to each cylinder of the engine. The pumps typically contain at least one pumping piston that is reciprocated by a cammed connection to the engine drive shaft. Common rail systems no longer require direct synchronization of a plurality of pumping pistons or rotating hydraulic head, with the injection events. Instead, the pump need only maintain the rail at the desired pressure. As a consequence, designers now favor single piston pumps for a wide variety of operating regimes.
The high pressure pump is typically fed from a low pressure feed line entering the charging line of the pump. Due to the high rate of piston reciprocation of a single piston to produce a sufficient number of discrete quantities of high pressure fuel discharged into the rail, the charging system of the pump can experience significant back pressure pulses which produce unwanted noise and wear on the pump internals. Although techniques are known for back pressure relief to a low pressure sink or auxiliary accumulator, vehicle manufacturers seeking to conserve space in the engine compartment and save cost, are increasingly requiring even single piston fuel pumps to have internal damping, i.e., without flow to a low pressure fuel reservoir or fuel accumulator.
U.S. Publication 2008/0175735 “Inlet Pressure Attenuator for Single Plunger Fuel Pump” and U.S. Pat. Nos. 7,401,594 and 7,124,738 (both titled “Damper Mechanism and High Pressure Fuel Pump”) describe techniques for internal damping of pressure pulsations in the low pressure side of a high pressure fuel pump. Two metal diaphragms are joined together around the circumference to envelope an internal volume of pressurized gas (“double diaphragm”). One or two such double diaphragm assemblies are situated in a damping chamber upstream of the inlet valve for the high pressure pumping piston.
The damper system of U.S. Pat. No. 7,124,738 has some inherent drawbacks. Firstly, two convoluted diaphragms are welded to each other on the outer periphery. This creates a condition where the weld strength is compromised. Because the diaphragms are made from a thin material, the resulting weld bead is of a small cross-section and the assembly (with thin capsule shaped diaphragms) has minimal hoop strength and resistance to flexing at the weld. This was overcome according to the '738 patent by clamping the diaphragms with significant force inboard of the weld bead. Secondly, the damper will be overstressed and fail in the convolutions if excess pressure pulsations are encountered due to activation of the high pressure system overpressure relief valve. When this relief valve is functioning, pressure pulsations of two to three times normal operation are encountered. Finally, when one of the flexible diaphragms does fail, the common volume of gas shared by the two diaphragms becomes filled with fluid, causing both diaphragms to become inoperable.